The present invention relates to an apparatus and method for fabricating bonded substrate (panel). More particularly, the present invention relates to an apparatus and method for fabricating bonded substrate for a liquid crystal display (LCD), which is provided by bonding two substrates at a predetermined gap.
Nowadays, there are demands for large and thin liquid crystal display (LCD) panels capable of providing fine display on a large display area, and apparatus which fabricates such LCD panels have been developed. An LCD panel is fabricated by arranging two glass substrates to face each other at an extremely narrow gap (several micrometers) and filling a liquid crystal between the two glass substrates. The two glass substrates are, for example, an array substrate on which a plurality of TFTs (Thin Film Transistors) are formed in a matrix form and a color filter substrate on which color filters (red, green and blue), a light shielding film, etc. are formed. The light shielding film contributes to improving contrast and shields light toward the TFTs to prevent generation of an optical leak current. The array substrate is bonded to the color filter substrate by a sealing material (adhesive) containing a thermosetting resin.
A method of fabricating an LCD panel includes a liquid crystal sealing step of sealing a liquid crystal between two glass substrates. The conventional liquid crystal sealing step is carried out by the following vacuum injection method. First, the TFT-formed array substrate is bonded to the color filter substrate (opposing substrate) via a sealing material. The sealing material is cured. The bonded substrates and a liquid crystal are placed in a vacuum tank and an inlet port provided in the sealing material is dipped in the liquid crystal. The pressure in the tank is set back to the atmospheric pressure so that the liquid crystal is sucked from the inlet port. Finally, the inlet port of the sealing material is sealed.
Recently, attention has been paid to the following dropping method instead of the vacuum injection method. First, the frame of a sealing material is formed in such a way as to enclose the outer periphery of the array substrate. A predetermined dose of a liquid crystal is dropped on the surface of the array substrate within the frame of the sealing material. Finally, the array substrate is bonded to the color filter substrate in vacuum. The dropping method can reduce the amount of a liquid crystal in use significantly and can shorten the time needed for the liquid crystal sealing step, thus resulting in a reduction in panel fabrication cost. It is therefore expected that mass production will be improved.
A bonded-substrate fabricating apparatus which operates according to the dropping method has the following problems.
1. Improper chuck originated from the bending of substrate
Normally, a substrate is held by suction by vacuum chuck or chucking by electrostatic chuck. In the vacuum-chuck holding, a holding plate which can hold a substrate by vacuum suction is used. The array substrate is held by the holding plate and the frame of the sealing material is formed on the array substrate. The adequate amount of a liquid crystal is dropped on the surface of the array substrate from a dispenser. Finally, the array substrate is bonded to the color filter substrate in a vacuum atmosphere.
In the electrostatic-chuck holding, a holding plate which has an electrode is used. A voltage is applied between the electrode of the holding plate and a conductive film formed on a glass substrate to generate Coulomb's force between the glass and the electrode. The Coulomb's force electrostatically holds the glass substrate on the holding plate.
In the vacuum-chuck holding, when the degree of vacuum in the process chamber becomes as high as a certain level, vacuum chuck does not work. In this respect, the substrate is electrostatically held by electrostatic chuck before suction by vacuum chuck stops working.
Normally, two substrates are separately held by an upper holding plate and a lower holding plate and are bonded together. Specifically, to prevent transfer of dust on the bonded surfaces or contamination thereof, the outer edge areas (portions outward of the frame of the sealing material) of the substrates are held by a transfer robot and are moved into the process chamber. However, the large and thin substrates are likely to curve (bend) due to their dead loads. The holding plates cannot stably hold the bent substrates. If the process chamber is depressurized for the purpose of bonding the substrates, therefore, the misalignment of the substrates or separation of the substrates from the holding plates may occur.
In a case where the holding plate (electrostatic chuck) electrostatically holds a bent substrate, glow discharge occurs during depressurization of the process chamber. This case brings about a problem such that a circuit or TFT devices formed on the substrate are damaged, resulting in generation of defects. In addition, as air remains between the holding plate and the substrate, the substrate may be released from the electrostatic chuck while depressurizing the process chamber.
2. Improper bonding originated from the bending of substrate
In the bonding step, two substrates are pressed while keeping a predetermined substrate gap. The important factors in the bonding step are to keep the two substrates parallel to each other and to press the two substrates with a uniform load. If the substrates are bent, however, the frame of the sealing material is pressed unevenly in the bonding step, so that the liquid crystal may be pushed out of the frame of the sealing material. If the pressing pressure is uneven, the pressing pressure needed to seal the liquid crystal increases so that the influence on the substrates becomes greater. This makes it difficult to fabricate stable products.
3. Dust oriented improper chuck
The holding plates that hold the two substrates separately have chuck surfaces which are planarized at a high precision. In a case where dust or glass pieces are adhered to the chuck surfaces, the dust is transferred onto the substrates, causing the misalignment of the substrates or separation of the substrates from the holding plates. As minute dust is adhered to the holding plates by electrostatic force, however, it is difficult to remove the dust from the holding plates.
4. Defects originated from variation in cell thickness
It is necessary to properly adjust the amount of a liquid crystal to be sealed in an extremely narrow substrate gap (cell thickness). The gap between the two substrates is determined by placing spacers between the substrates or forming poles on one of the substrates. However, the spacers and poles have a slight height variation. This results in a change in the substrate gap, so that the amount of the liquid crystal sealed may become too much or too little locally. This would bring about a problem such that the cell thickness would vary after the substrates were bonded. The variation in cell thickness cause uneven display of the LCD panel.